Shows how to dynamically implement a class Person using ByteBudy

DynamicPersonClass.kt

package org.example

import net.bytebuddy.ByteBuddy
import net.bytebuddy.description.modifier.Visibility
import net.bytebuddy.dynamic.scaffold.subclass.ConstructorStrategy
import net.bytebuddy.implementation.FieldAccessor
import net.bytebuddy.implementation.MethodCall
import net.bytebuddy.implementation.MethodDelegation
import net.bytebuddy.implementation.bind.annotation.FieldValue
import net.bytebuddy.matcher.ElementMatchers

val personClass = ByteBuddy()
    .subclass(Any::class.java, ConstructorStrategy.Default.NO_CONSTRUCTORS)
    .name("Person")
    .defineField("firstName", String::class.java, Visibility.PRIVATE)
    .defineField("lastName", String::class.java, Visibility.PRIVATE)
    .defineConstructor(Visibility.PUBLIC)
    .withParameters(String::class.java, String::class.java)
    .intercept(
        MethodCall.invoke(Any::class.java.getDeclaredConstructor())
            .andThen(FieldAccessor.ofField("firstName").setsArgumentAt(0))
            .andThen(FieldAccessor.ofField("lastName").setsArgumentAt(1))
    )
    .method(ElementMatchers.isToString())
    .intercept(MethodDelegation.to(PersonToString::class.java))
    .make()
    .load(ClassLoader.getSystemClassLoader())
    .loaded as Class<*>

object PersonToString {
    @JvmStatic
    fun stringuifyPerson(
        @FieldValue("firstName") fn: String, 
        @FieldValue("lastName") ln: String): String {
        return "$fn $ln is the person's name"
    }
}

fun main() {
    val person = personClass
        .getDeclaredConstructor(String::class.java, String::class.java)
        .newInstance("John", "Smith")
    println(person::class.java.name)
    println(personClass.declaredFields.asList())
    println(person.toString())
}